1. Field of the Invention
The present invention relates to a method for manufacturing a green compact useful as a material for a high frequency dust core by compacting a soft magnetic powder such as an iron powder or an iron base alloy powder. More particularly, it relates to a compaction method capable of enhancing the green density, and further improving the mechanical properties, magnetic properties, and the like of the final green compact.
2. Description of the Prior Art
In recent years, there has been used a green compact obtained by compacting a soft magnetic powder such as an iron powder or an iron base alloy powder (below, may be typically referred to as xe2x80x9ciron powderxe2x80x9d) as a material for a high frequency dust core. In order to enhance the mechanical properties and magnetic properties of such a green compact, it is important to increase the density and the strength as high as possible. Various technologies have been proposed heretofore from such a viewpoint of attaining higher density and higher strength.
For example, JP-A-No. 50138/1984 proposes a dust core material obtained by coating each particle of an iron powder with an organic binder such as an epoxy resin or a fluorocarbonresin. With this technology, the strength can be improved to a certain degree by mixing a resin therein. However, the improvement ratio of the strength is determined by the characteristics of the resin itself. Therefore, the resulting strength does not reach the level capable of sufficiently satisfying the recent demand for higher strength. Whereas, when a resin is mixed with the iron powder, the volume fraction of the iron powder decreases by the amount of the resin mixed. Accordingly, the green density decreases at least by the amount of the resin added relative to the green density for 100% iron powder. The resin is required to be added in such an amount as to sufficiently coat around the particles of the iron powder for improving the strength. However, the density of the green compact decreases correspondingly, so that the magnetic properties of the green compact such as the magnetic flux density or the magnetic permeability remain unsatisfactory.
Whereas, in JP-A-No. 245209/1995, there is proposed a technology for improving the strength of a green compact by using an iron powder which has been subjected to surface phosphating in place of the organic binder coating used in JP-A-No. 50138/1984. However, with this technology, the strength is improved more than when only an iron powder is compacted due to the effect of the phosphating treatment. Nevertheless, the effect of the strength improvement is less as compared with the case where an organic binder is coated thereon as disclosed in JP-A-No. 50138/1984. Further, with the compaction method as disclosed in JP-A-NO. 245209/1995, a lubricant is required to be pre-mixed with an iron powder from the viewpoint of preventing the seizure between a green compact and a compaction die. Accordingly, the volume fraction of the iron powder decreases by the amount of the lubricant added. As a result, the green density decreases at least by the amount of the lubricant added relative to the green density for 100% iron powder. Therefore, this technology is not sufficiently adaptable to the recent demand for further higher density and higher strength, either.
Further, for example, in JP-A-No. 272901/1997, there is also proposed a technology in which a lubricant is not mixed with an iron powder, but applied onto only the inner wall surface of a compaction die, followed by (warm) compaction at a temperature of from 150 to 400xc2x0 C. This method is a so-called die wall lubrication method. The iron powder basically contains no lubricant not resin for coating the organic binder, resulting in no occurrence of the reduction in green density due to mixing thereof as described above. However, even if such a die wall lubrication method is applied to a conventional soft magnetic powder, in actuality, the strength improvement is not achieved as much as expected.
On the other hand, a study has also already been underway from the viewpoint of improving the magnetic properties of the green compact. For example, such a technology as disclosed in Japan Patent No. 2710152 is proposed. With this technology, the particles of an iron powder individually coated with an insulating vitreous layer containing P, Mg, B, and Fe as essential components are used. The starting powder and a lubricant are mixed for compaction, followed by annealing at a temperature of from 400 to 600xc2x0 C. for achieving joining between the insulating vitreous layers, thereby improving the insulating property and the magnetic flux density. Japan Patent No. 2710152 discloses that the strength improvement can also be accomplished by mixing a resin therewith as shown in JP-A-No. 50138/1984 in addition to coating thereof with the insulating vitreous layer in such a manner.
Subsequent study by the present inventors proves as follows. In the technology of Japan Patent No. 2710152, in addition to the strength improvement effect due to mixing of a resin, the insulating vitreous layers join together during the process of from compaction to annealing to contribute to the strength improvement of the green compact. As a result, the green compact strength is improved more than when only a resin is mixed. However, since the lubricant and the resin are mixed therein, the volume fractions of the iron powder decreases by at least the amount thereof. As a result, the green density decreases relative to the green density for 100% iron powder, still resulting in unsatisfactory circumstances for responding to the recent demand for further higher density. Further, with the technology of Japan Patent No. 2710152, annealing after compaction is carried out at from 400 to 600xc2x0 C. However, when annealing is carried out at such a high temperature, joining between the insulating vitreous layers further proceeds to improve the strength of the green compact, but the electric resistance is decreased as described below, presenting another problem that the resulting green compact is not applicable to a part required to have a high electric resistance.
On the other hand, other than the foregoing technologies, there is also proposed a technology of promoting densification of a powder by applying vibrations to the powder at a stage of compaction (ex. JP-B-Nos. 25278/1991, 6549/1966, and 5414781/1966). Further, the present inventors also proposes that use of an iron powder flattened so that the ratio of the mean particle size to the thickness is 4 or more is effective for high densification (JP-A-No. 260114/1996). However, only these technologies are insufficient for accomplishing higher density and higher strength of the green compact.
The present invention has been achieved in view of the foregoing circumstances. It is therefore an object of the present invention to provide a method for the compaction of a soft magnetic powder capable of manufacturing a green compact which as attained higher density and higher strength than ever without causing a reduction in electric resistance.
The present invention which has attained the foregoing object pertains to a method for the compaction of a soft magnetic powder, comprising: applying a lubricant to the inner wall surface of a compaction die, and subjecting the soft magnetic powder to compaction at from not less than room temperature to less than 50xc2x0 C. without mixing the lubricant with the soft magnetic powder, and then annealing a resulting green compact at from 50 to 300xc2x0 C., particles of the soft magnetic powder being individually surface-coated with an insulating vitreous layer containing P, Mg, B, and Fe as essential components.
The object of the present invention can also be attained by using soft magnetic powder particles individually surface-coated with an insulating vitreous layer containing P, Mg, B, and Fe as essential components, applying a lubricant to the inner wall surface of a compaction die, and subjecting the soft magnetic powder to compaction at from not less than 50xc2x0 C. to less than 250xc2x0 C. without mixing the lubricant with the soft magnetic powder.
In the method of the present invention, it is appropriate that the pressure at the time of compaction is from 250 to 1500 MPa. If the maximum pressure at the time of compaction is set at from 500 to 1500 MPa, and vibrations are applied to the compaction die, the vibration in a pressure-free condition is set to have a single amplitude of 0.002 to 0.20 mm, and the amplitude of the vibration for all or a part of the time during which the compaction pressure is 500 MPa is not less than 20% of the amplitude in the pressure-free condition, preferably, higher densification of the green compact is attained. In such a method in which vibrations are applied, it is preferable that the frequency of the vibration is set at from 5 Hz to 20 kHz. Further, the soft magnetic powder to be used in the present invention preferably has a ratio (d/t) of the mean particle size d to the thickness t of not less than 4.